This invention relates to a pressing-in device for a pressure die casting machine, comprising a casting piston for pressing casting material into the mold of a pressure die casting machine.
Modern pressing-in devices of pressure die casting machines are conceived as multi-phase pressing-in systems, particularly three-phase systems. In this case, in a first slow lead phase, the casting piston is moved forward until the casting material arrives at the gate, whereas, in the second phase, it is pressed into the mold cavity of the closing part with a short filling time and at a high pressing-in speed. In a third phase, a compression of the die-cast part which is decisive for the quality of the cast products takes place under high pressures. For implementing these phases, the casting piston is connected with a high-cost hydraulic system which comprises several pressure accumulators as well as control and regulating devices.
A pressing-in device of the above-mentioned type is known from German Patent document DE 29 22 914 C2. In the case of this known pressing-in device, three pressure accumulators are provided. The pressure accumulators are connected with the casting piston cylinder via various lines provided with return valves, seat valves and control valves. In this case, the pressure medium from the first pressure accumulator moves the casting piston from an inoperative position into a position in which the casting material is at the gate, whereupon, after the change-over of a seat valve, the pressure medium is guided out of the second pressure accumulator under high pressure and at a high speed onto the casting piston so that the casting material is rapidly injected into the mold. For the implementation of the third phase, another seat valve is then switched. This seat valve is connected in front of the third pressure accumulator so that the pressure existing in the third pressure accumulator can be transmitted to a multiplier piston and can be transmitted by this multiplier piston to the casting piston while utilizing a transmission ratio. As a result, the casting piston, which is in its end position, subjects the casting material already injected into the mold to a very high pressure. Thereby, high quality casting is achieved. In this case, the control of the different pressure phases is carried out as a function of the path covered by the casting piston. The position of the piston rod of the casting piston must therefore be monitored by separate devices. In the case of these constructions, it is a disadvantage that, because of the high temperatures in the area of the pressure die casting machines, so-called water glycols are used instead of oil as the hydraulic fluid. These water glycols are not as flammable as oil, but are more harmful to the environment and, if any seals are damaged, may also harm the operator's health.
For machine tools, and also for screw presses (see: Dubbel, "Taschenbuch fur den Maschinenbau" ("Manual for Machine Construction"), 14th Edition, Springer Publishers '81, Pages 983 and 9804), it is known to use electric motors as drives. However, different conditions exist in the above machine tools than in the case of pressing-in devices for pressure die casting machines. For the above machine tools, for example, the feed motion of the pressing head may be carried out at a relatively high adjusting speed. However, in this case, the spindle does not have to apply large forces. If such large forces become necessary during the deformation operation, the spindles run relatively slowly. Because of the engaging of transmissions, commercially available electric motors may therefore be used. This is not possible in the case of pressure die casting machines because of the required injection speeds for the casting material and because of the high pressures which must therefore be exercised.
There is therefore needed a pressing-in device of the above-mentioned type wherein the use of a hydraulic fluid is no longer necessary, without any impairment of the prerequisites which exist for the pressure die casting operation.
For achieving this need, at least one electric motor is provided as a drive for the casting piston. Because of the use of an electric motor as the drive of the casting piston, the casting piston movement can be controlled in a precise manner. The construction of the pressing-in device is simplified because a high-cost hydraulic system is no longer necessary. The use of a hydraulic fluid, which is harmful to the environment, is avoided. Finally, the rotating movement of the electric motor may also directly be utilized for the control of the casting operation. The respective position of the piston rod of the casting piston can be determined from the number of revolutions. In this case, it was found to be particularly advantageous and suitable for the drive of the pressing-in device of a pressure die casting machine to use an electric motor which drives the casting piston via a spindle or a toothed rack, in which case it is particularly advantageous to use a recirculating ball screw as a spindle. In the case of this development, high advancing speeds are implemented in the case of very high pressures, as required for pressure die casting machines and as explained above.
The use of an electric motor becomes possible particularly because the sensitive thread portion of the spindle, which extends within the nut provided in the case of a recirculating ball screw, is completely encapsulated and therefore outwardly protected. The contaminations which occur in the rough casting operation and which are otherwise hard to keep away from the drive, will have no damaging influence on the drive in this manner.
A further development provides a gear reduction between the electric motor and the casting piston. As a result, different pressures can be achieved because of different torque forces, so that the drive can also be used in the case of larger machines.
In the case of another embodiment, the electric motor may be a multispeed electric motor permitting the initially mentioned output adaptation required in the case of the pressing device of pressure die casting machines with respect to the speeds as well as with respect to the forces to be generated.
Finally, water cooling may be provided for the electric motor itself, in the event its power has to be increased.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.